The realization of a complete silicon based photonics platform requires the co-integration of advanced passive photonic devices, e.g., using poly-on-SOI (such as, for example, high-efficiency raised grating couplers, filters, polarization rotators and/or waveguides), advanced electro-optic modulators and switches (such as, for example, semiconductor-oxide-semiconductor (SOSCAP) modulators, e.g., employing the gate-oxide capacitance between poly and SOI, and/or p-n diodes for carrier depletion and injection), hybrid III-V/Si lasers and amplifiers using bonding technology, and germanium-on-silicon (Ge-on-Si) waveguide photodetectors.
In hybrid III-V/silicon lasers and amplifiers, optical gain is achieved in a low-defect III-V layer that is first grown epitaxially on a separate substrate and then bonded to, e.g., a SOI substrate. In order to allow efficient evanescent coupling between the silicon layer and the III-V layer, both layers are in close proximity, such as at a distance less than 100 nm. Bonding of the III-V layer with thin interfacial layers requires a flat surface with a low topography.
The co-integrated germanium-on-silicon (Ge-on-Si) waveguide photodetectors preferably have a good performance, such as a low dark current, requiring a low defect density of threading dislocations (less than 107 per cm2); a high responsivity in the order of 1 A/W, requiring efficient optical coupling between silicon and germanium, low parasitic absorption at the metal contacts and efficient carrier collection; and a high speed, e.g., in the range between 10 GHz and 40 GHz, requiring thin layers and a small footprint.
Germanium waveguide photodetectors can, for example, be integrated with a silicon photonics platform by transferring and wafer bonding a crystalline germanium film onto a silicon based (e.g., SOI) wafer, and then fabricating the photodetector in the transferred film. However, using a wafer bonding approach is relatively expensive and makes co-integration with other opto-electronic components difficult, in particular the co-integration with hybrid III-V/silicon lasers, requiring bonding of III-V layers in close proximity to a silicon waveguide.
Germanium waveguide photodetectors can also be integrated with a silicon photonics platform by growing (e.g., epitaxially) a germanium layer on top of a silicon waveguide and then fabricating a germanium photodetector. In order to enable co-integration with other opto-electronic components and, e.g., hybrid lasers, a planar topology is needed, requiring, for example, a thin germanium film for the photodetectors (e.g., thinner than 300 nm). However, metal contacts on top of such a thin film are known to cause excessive optical absorption, which detracts from the photocurrent and, as such, negatively affects the photodetector responsivity. These negative effects are even more pronounced in a thin germanium layer epitaxially grown on top of silicon than for a thicker germanium layer.